Coating of fibrous structrues with tetrafluoroethylene polymer



March 1959 M. c. KUMNICK ETAL COATING 0F FIBROUS STRUCTURES WITH TETRAFLUOROETHYLENE POLYMER Filed Nov. 23, 1954 222:2 Q MIUJZEZZAUZELCJZ e e o o INVENTOR5 MILES C. KUMNICK JOHN F. LONTZ ATTORNEY United States Patent COATING OF FIBROUS STRUCTURES H TETRAFLUOROETHYLENE POLYMER Miles Charles Kumnick and John Frank Lontz, Wilmingin, Del., assignors to E. l. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application November 23, 1954, Serial No. 470,813

4 Claims. (Cl. 117-47) This invention relates to the coating of fibrous structures with tetrafiuoroethylene polymer and more particularly to coating of fibrous structures using aqueous gels of tetrafiuoroethylene polymer.

Polymerized tetrafiuoroethylene and a method for pre' paring it are disclosed and claimed in U. S. Patent 2,230,654. Polytetrafiuoroethylene has become well known for its high chemical inertness and its high temperature applications. More recently the low coefficient of friction and low adhesion of tetrafiuoroethylene polymer have become known and are described in scientific papers such as by K. V. Shooter in Proc. Rog. Soc. (A) 213,488 (1952) and Research 2,533 (1949).

Because of these properties tetrafiuoroethylene polymers are highly desired for coating of surfaces. The extremely high melting point of polytetrailuoroethylene, its insolubility in all common chemical solvents, and its extremely high melt viscosity have led to the application of colloidal aqueous dispersions of polytetrafiuoroethyl ene or modifications thereof in coating uses. The preparation of aqueous dispersions of tetrafiuoroethylene polymer is well known to those skilled in the art, having been described in U. S. Patents 2,393,967, 2,559,752, and 2,612,484 among others. The application of modified aqueous dispersions of tetrafiuoroethylene for coating purposes has been described in U. S. Patents 2,532,691 and 2,613,193 among others. These aqueous dispersions are well suited for applying coatings to solid surfaces such as metals or to impregnate fibrous structures with polytetrafiuoroethylene. These aqueous dispersions however are not well suited for applying coatings of polytetrafluoroethylene to fibrous structures since these are easily penetrated by aqueous dispersions of the polymer, distributing the polymer throughout the fibrous structure instead of concentrating the polymer on the surface. Furthermore, in many cases it is desired to have only one surface of the fibrous structure coated, a process which could be obtained only with great difficulty for fibrous materials by prior art processes.

It is therefore the principal object of this invention to provide a process which will permit the formation of dense, stronglyadhering polytetrafiuoroethylene coatings on fibrous material, causing only a surface penetration of said fibrous structures.- Another object of this invention is to provide surfaces having low adhesive properties. Still another object is to provide surfaces of low coefficient of friction superimposed and strongly adherent to resilient fibrous structures. Other objects will appear hereinafter.

These objects are accomplished in accordance with this invention by coating fibrous structures with an aqueous colloidal gel, obtained by admixing an aqueous colloidal dispersion of tetrafluoroethylene polymer with from to 40% by weight of said dispersion of a liquid organic compound boiling below 200 C. and being miscible with water in all proportions and baking the resulting surfaceimpregnated fibrous structure above 100' C. and below the decomposition temperature of the fibrous material 2,876,131 Patented Mar. 3, 1959 until all of the volatile components of the gel are removed.

It has now been discovered that stable gels of the tetrafiuoroethylene polymer in water can be prepared by adding with slight agitation water-miscible oxygenated aliphatic compounds comprising alcohols, ketones, and ethers. These gels are stable over long periods of time and can be spread evenly on fibrous surfaces as continuous coatings. The polytetrafluoroethylene particles are suspended in the liquid phase in a partially coagulated state in a flexible lattice structure and thus give rise to viscosities of 10,000 centipoises and higher. When a viscous gel of such a nature is applied to or formed in situ on a fibrous material no significant penetration of said fibrous material occurs, thus concentrating the polymer on the surface. The gel however is fluid enough to cause a surface penetration of the fibrous structure thus providing for adherence of the-gel and the coating resulting therefrom to the surface of the material. In the drying operation the partially coagulated polymer particles will then coagulate completely in and on the surface of the fibrous structure, thus adhering strongly to the fibrous material, so that no further sintering operation is necessary to assure good adhesion, which in many applications would destroy the fibrous structure due to the high temperatures involved. However, should the fibrous structure be sufficiently heat stable the polymer particles may be sintered to further increase the adhesion between fiber and coating.

The gel used in the application of this invention is formed preferably by addition of the organic liquid to the aqueous dispersion of tetrafluoroethylene polymer. The reverse addition may lead to complete coagulation of the polymer in the dispersion, i. e., the solid polymer phase will separate from the liquid phase. The function of the organic solvent is two-fold. One, it acts as a lubricant to prevent complete coagulation during the application step, and two, it causes the formation of the gel. The amount of organic solvent added will depend on the concentration of the polytetrafiuoroethylene in the aqueous phase and the type of organic solvent used. The formation of the gel occurs instantaneously when a certain ratio of polymer to organic solvent to water is obtained. Prior to this critical point the aqueous dispersion of tetrafiuoroethylene polymer will not show any significant increase in viscosity on addition of the organic solvent. The addition of excess organic solvent will cause complete coagulation of the polymer, resulting in the separation of the liquid from the solid polymer phase. The amount of the organic solvent to be added to the aqueous dispersion of tetrafiuoroethylene polymer to cause the formation of the gel decreases as the concentration of the polymer in the aqueous phase increases and is easily determined experimentally on small samples. The viscosities of the gels obtained by adding the right amounts of organic solvents to aqueous tetrafiuoroethylene dispersions will also vary with the concentration of the polymer in the dispersion. As the concentration of the tetrafiuoroethylene polymer in the dispersion is increased the viscosity of the gel resulting on addition of the organic liquid will increase. Thus by adjusting the solids concentration of the dispersion a large variety of gels can be prepared to obtain coatings most desirable. The formation of the gel, used in the process of this invention, occurs rapidly upon addition of the correct amount of the organic phase. The addition of the organic solvent is carried out at room temperatures to avoid complete coagulation of the dispersion occurring at higher temperatures.

Dispersing agents present in aqueous dispersions of tetrafiuoroethylene polymer will not prevent the formation of the gel. It is believed that the dispersing agents contribute to the stability of the resulting gel, and thus aid in the formation of a gel making the addition of the organic solvent less critical.

A large class of organic compounds may be used to cause the gelling action of the aqueous dispersion of tetrafluoroethylene polymers and give a stable gel. This class of compounds, however, must meet the following specification: (l) the solvent to be used must be water miscible, (2) the polymer must not be wetted by the solvent nor plasticized by it, (3) the solvent must boil below the decomposition temperature of the fibrous material. Preferred solvents are alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, ketones such as acetone, and ethers such as ethyl ether. The relatively inexpensive ketones, especially acetone, are for economical reasons as well as reasons of solubility and low boiling temperatures as outlined above, especially preferred.

The attached drawings, Figure 1 and Figure 2, show schematically two methods of applying the polytetrafiuoroethylene gel to a fibrous surface.

The gel may be formed in a separate container and then applied to a fibrous material by a calendering technique or with the aid of a roll or a doctor blade, as schematically shown in Figure l. The gel may be also formed in situ. This is accomplished by wetting the fibrous material to a certain extent as determined by weight and then coating the surface desired with an aqueous dispersion of tetrafluoroethylene polymer having a specified solids content. This may be then followed by rolling under slight pressure to compact the gel on the surface of the fibrous material, as schematically shown by Figure 2.

Due to the fact that the viscosity of the gel used in the process of this invention can be varied to obtain the best results, it is possible to use this coating process for a wide variety of fibrous structures. These materials may be unwoven structures such as felt, asbestos sheeting or glass mat or woven materials such as canvas, wool, cotton and synthetic fiber fabrics. In some instances it may be desirable to increase the adhesion of the tetrafluoroethylene polymer particles to fibers by prior coating or impregnation of the fibers with compounds which will increase said adhesion. In order to obtain good adhesion between the surface of the fabric and the polytetrafluoroethylene coating, some surface penetration should be possible. Thus the adhesion to a pore-free surface will not be as good as to one that will permit surface penetration. The coating obtained by the process of this invention is in many instances not a continuous film, but surface fibers and strands are coated by the polymer, thus imparting the desired surface properties, without affecting the general properties of the fibrous material such as resiliency and flexibility. Shrinkage of the fibrous material, upon being treated in accordance with the process of this invention, is small and generally can be disregarded.

The following examples are given to further illustrate but not to limit the process of this invention. All parts are by weight unless otherwise specified.

Example I.-To 75 parts of an aqueous dispersion of tetrafluoroethylene resin containing 17.5 parts of colloidal tetrafluoroethylene polymer and 1.05 parts of a dispersing agent (Triton X-l), which is a non-ionic surface-active agent that is chemically an alkyl aryl polyether alcohol was added under mild agitation, 25 parts of isopropylalcohol. Upon agitating for a period of five minutes a gel formed having a viscosity of approximately 10,000 centipoises. This gel was then applied to a woven felt 20-ounce, inch fabric which had previously been dried at 105 C. with the aid of a doctor blade. The felt material was rolled several times with a hard rubber roll to insure good surface impregnation. The coated fabric was then passed through calendering rolls at 160 C. until all the alcohol and water had been removed. Enlarged cross sectional photographs of the coated felt fabric showed that the coating had not penetrated beyond a depth of 20 mils.

Example 2.Samples of felt material $5 inch thick dried at 105 C. until a constant weight was obtained were dipped into acetone and drained of excess acetone until an acetone pickup of approximately 150% was ob tained. The wet felt material was then dipped for a period of one minute into an aqueous dispersion of polymeric tetrafluoroethylene at room temperature containing 36% by weight of the water of tetrafluoroethylene polymer and 2.1% by weight of the water of a dispersing agent. The acetone present on the surface of the felt material partially coagulated the tetrafluoroethylene polymer at the surface of the felt material forming the desired gel. The felt material having the tetrafluoroethylene polymer gel on the surface was passed through hard rubber rolls under slight pressure and then dried in an oven at 105 C. until all water and acetone had been removed, as indicated by constant weight of the surface impregnated material.

Example 3.-To parts of an aqueous dispersion of colloidal tetrafluoroethylene polymer containing 60% by weight of the water of tetrafluoroethylene polymer and 6% by weight of the water of a dispersing agent, was added under mild agitation and dropwise 23.5 parts of isopropyl alcohol. Upon complete addition of the alcohol the viscosity of the mixture increased rapidly until a thick gel was formed having a viscosity of approximately 500,000 to 1,000,000 centipoises. This gel was then applied to a canvas belting material with the aid of a doctor blade until a smooth coating had been obtained. The coated material was then dried in an oven at a temperature of C. until a constant weight was obtained, indicating the complete removal of Water and alcohol. The underside of the coated material was completely free of tetrafluoroethylene polymer and retained the roughness needed for traction, as a conveyor belt.

Example 4.Samples of a wool fabric 3 inches square and $1 inch thick were dried at 105 C. until a constant weight was obtained and then dipped into isopropyl alcohol and drained of excess isopropyl alcohol until an isopropyl alcohol pickup of approximately was obtained. The wet felt material was then dipped for a period of one minute into an aqueous dispersion of tetrafluoroethylene polymer containing 36% by weight of the water of tetrafluoroethylene polymer and 3.6% by weight of the water of a dispersing agent. The isopropyl alcohol present on the surface of the wool fabric partially coagulated the colloidal tetrafluoroethylene polymer at the surface of the woolen fabric forming the desired gel. The woolen fabric with the polymer gel on the surface was dried at 105 C. until all water and alcohol had been removed as indicated by constant weight of the surface impregnated material. Cross sectional photographs showed that no major penetration of the wool fabric had occurred in this coating process.

Example 5.To 68 parts of an aqueous dispersion polytetrafluoroethylene containing 41 parts by weight of the polymer and 2.4 parts by weight of Triton X-100," 32 parts by weight of dioxane were added under mild agitation. The resulting gel was applied to a M inch asbestos sheeting with the aid of a doctor blade. The asbestos sheeting coated with the gel was dried at 105 until all water and alcohol had been removed as indicated by constant weight of the surface impregnated material.

The coated materials in the process of this invention are useful in applications where low adhesion and low coefficients of friction are desired; thus the polytetrafiuoroethylene coated felt is particularly useful in thegrooves of sliding windows as used in automobiles. Although impregnated polytetrafluoroethylene felt obtained from the aqueous dispersion of polytetrafluoroethylene has been used for such applications, the coated felts of this invention retain the resiliency of the fabric and also give optimum surface conditions for sliding glass panes.

Furthermore, canvas hoses used in the transportation of small solid particles may be coated on the interior with tetrafluoroethylene resin to prevent adhesion leading to plugging of the lines. Canvas belt conveyors coated with tetratluoroethylene polymer used in the transportation of resins and materials having tendencies to adhere to the belt will have little or no tendency to adhere to the conveyor belt and thus facilitate removal of said articles or materials from the belt. Resilient fibrous materials used as gaskets and washers coated with tetrafluoroethylene resin will have less friction and show less wear than uncoated material and thus had improved service life and utility. Other applications will be apparent to those skilled in the art.

We claim:

1. A process for obtaining fibrous structures coated with tetrafluoroethylene polymer which comprises coating said structures with an aqueous colloidal gel, obtained by adding to an aqueous colloidal dispersion of tetrafiuoroethylene polymer, containing from 10 to 50% of the tetrafluoroethylene polymer, based on the weight of the water, said dispersion containing an alkyl aryl polyether alcohol dispersing agent, from 10 to 25%, based on the weight of said dispersion, of a water-miscible oxygenated aliphatic compound boiling below 200 C. and baking the resulting surface impregnated fibrous structure above 100 C. and below the decomposition temperature of the fibrous material until all of the solvent is removed.

2. A process as set forth in claim 1 in which the organic compound is isopropyl alcohol.

3. A process as set forth in claim 1 in which the organic compound is acetone.

4. A process for obtaining polytetrafluoroethylenecoated fibrous structures comprising wetting said fibrous structure with a water-miscible oxygenated aliphatic compound boiling below 200 C., passing said wetted fibrous structure into an aqueous colloidal dispersion of polytetrafluoroethylene containing from 10 to 50% polytetrafluoroethylene based on the weight of the water, said dispersion containing an alkyl aryl polyethcr alcohol dispersing agent, and thereafter heating said fibrous structure at a temperature above 100 C. until all the water is removed, whereby a fibrous structure having a surface coating of polytetrafiuoroethylene is obtained.

References Cited in the file of this patent UNITED STATES PATENTS 2,311,488 Thomas Feb. 16, 1943 2,424,386 Herbert July 22, 1947 2,581,454 Sprung Jan. 8, 1952 2,613,193 Osdal Oct. 7, 1952 2,681,324 Hochberg June 15, 1954 2,686,767 Green Aug. 17, 1954 2,764,506 Piccard Sept. 25, 1956 

1. A PROCESS FOR OBTAINING FIBROUS STRUCTURES COATED WITH TETRAFLUOROETHYLENE POLYMER WHICH COMPRISES COATING SAID STRUCTURES WITH AN AQUEOUS COLLOIDAL GEL, OBTAINED BY ADDING TO AN AQUEOUS COLLOIDAL DISPERSION OF TETRAFLUOROETHYLENE POLYMFER, CONTAINING FROM 10 TO 50% OF THE TETRAFLUORETHLENE POLYMER, BASED ON THE WEIGHT OF THE WATER, SAID DISPERSION CONTAINING AN ALKYL ARYL POLYETHER ALCOHOL DISPERSING-AGENT, FROM 10 TO 25%, BASED ON THE WEIGHT OF SAID DISPERSION, OF A WATER-MISCIBLE OXYGENATED ALIPHATIC CMPOUND BOILING BELOW 200*C. AND BAKING THE RESULTING SURFACE IMPREGNATED FIBROUS STRUCTURE ABOVE 100*C. AND BELOW THE DECOMPOSITION TEMPERATURE OF THE FIBROUS MATERIAL UNTIL ALL OF THE SOLVENT IS REMOVED. 